DISK DRIVE CONTROL: THE EARLY YEARS
Daniel Abramovitch ∗ and Gene Franklin ∗∗
∗
Agilent Laboratories, Communications and Optics
Research Lab, 3500 Deer Creek Road, M/S: 25U-9, Palo
Alto, CA 94304, Phone: (650) 485-3806, FAX: (650)
485-4080, E-mail: danny@labs.agilent.com
∗∗
Stanford University, Information Systems Lab, 252
Packard Electrical Engineering Building, 350 Serra Mall,
Stanford, CA 94305, Phone: (650) 723-4837, E-mail:
franklin@isl.stanford.edu
Abstract: One of the persistently exciting control applications is that of disk drive
servos. From the start in the early 1950s to the massive capacity commodity drives
of the early 2000s, the problem of accessing data on rotating disk media has
provided a wealth of control challenges to be solved. This survey paper traces
the early history of disk drive control from the first disk drive in 1956 to the first
commercial drive with Magneto-Resistive heads in 1990. Rather than the approach
used in (Abramovitch and Franklin, 2002) in which the histories of the components
were outlined first, we will focus on the feedback loop itself in those early days.
The paper will survey the different areas of the disk drive control problem and
how they evolved.
Keywords: Disk drive control, early history, servomechanisms, computer
hardware
1. INTRODUCTION
One of the persistently exciting control applications is that of disk drive servos. From the start in
the early 1950s to the massive capacity commodity drives of the early 2000s, the problem of accessing data on rotating disk media has provided
a wealth of control challenges to be solved. This
shows no signs of abating as storage densities,
capacity and transfer rates keep rising while costs
and size keep dropping (Porter, 1998). Although a
host of new technologies (Magnetic RAM, Atomic
Resolution Storage, optical storage) are persistently poised to challenge the supremacy of hard
disks in their primary purpose of providing vast
storage at low cost, reports of the latter’s demise
are consistently and greatly exaggerated.
The purpose of this paper is to provide a history of control in the early days of disk drives.
While this subject can include both flexible and
optical drives, this paper will focus on rigid magnetic disks – often called hard disks. For a short
general history of disk drives culled from several recent excellent sources (Stevens, 1997; Rostky, 1998; Porter, 1996; Porter, 1998) can be found
in (Abramovitch and Franklin, 2002). In the latter
we presented an outline of the history of disk drive
control. In this paper, we will focus on the early
years of the drive control problem. The question of
what constitutes “early” is a natural one at this
point. We have chosen 1990, the year that IBM
came out with the 681 Redwing drive, the first
commercial drive based on the Magneto-Resistive
(MR) head (Quantum Staff, 2000).
This drive is chosen as a breakpoint not due to its
technical or commercial success (which was considered quite low a the time), but because as the
first commercial MR head drive it for-shadowed
the future of disk drives. While many drive companies had MR head drives in the lab, the leap to a
commercial MR head drive was at once necessary
and risky. Seagate would not produce a MR head
drive until late 1996. HP would exit the drive
business trying to produce their first MR head
drives. However, as MR heads changed the slope
of areal density improvement from 30% per year
to 60% per year, they dramatically changed what
the disk drive market by the year 2001. Thus, the
first MR head commercial drive is our break point.
2. A WALK AROUND THE LOOP
Position
Magnetic
Sensing
Media
Spindle Noise
Dynamics
S
-
Disk Position
DAC
Noise
ADC
PES Noise
Demod
ADC
DAC
PA
Noise
Power
Amp
DSP
Windage
(Air Flow)
LDV
Dedicated
Servo
Track
Eccentricity
Sectored
Servo
Fig. 2. Disk stacks for dedicated and sectored
servo systems. The dedicated servo on the left
allows the entire surface to be covered with
position information, thus enabling a higher
sample rate. The sectored servo in the center
multiplexes the position information with the
user data, which puts a practical limit on
the sample rate. However, it does co-locate
the position information with the user data
making the servo system largely immune to
mechanical offsets in the actuator assembly.
Both types of formatting are subject to track
eccentricity, shown on the right, where the
tracks can be non-circular and not properly
centered.
S
Head Position
Fig. 1. Generalized view of track following model.
A schematic block diagram of a disk drive control
loop is shown in Figure 1. The disk loop starts
with the disks stack assembly diagrammed in
Figure 2: a stacking of magnetic disks on a spindle
with an internal spindle motor. The magnetic
media contains data in concentric circular tracks
on both sides of the media.
Modern disk drives read the relative position of
the head to the track directly from the disk media.
Virtually all of today’s drives use a method called
sectored servo, in which user data and position
information are multiplexed in space around the
disk. As the drive spins, this spatial multiplexing
becomes a temporal multiplexing.
The data read heads are used to read position and
data are universally based on magnetoresistive
head technology, which presents some interesting
servo challenges. The position information takes
the form of a signal modulated into magnetic
domains – shown in Figure 3 – and this signal
must be demodulated. From there, the data is
digitized and fed into a digital processor, most
often a DSP, for implementation of the control
law. The output of the processor is converted back
to an analog signal and sent to a power amplifier
Disk
Surface
Arm Mechanics
B Field
Read
Head
A Field
Readback
Signal
Motor
Torque
A Burst
B Burst
Fig. 3. A simplified view of split field amplitude
encoded servo fields on a hard disk and the
resulting readback signals. In split field encoding the A fields and B fields are separated
down the track. Amplitude/area estimates of
the A and B fields, Ā and B̄, are computed
separately and subtracted from each other.
which drives a rotary voice coil actuator, shown
in Figure 4d. The rotary voice coil actuator moves
the magnetic heads through a suspension designed
to minimize the effect of the drive mechanics on
the servo loop. The suspension also provides a
preload to press the sliders down towards the disk
in opposition to the air bearing being generated
by the spinning disk. At the bottom of the slider
is the magnetic read/write element. In current
disk drives, this is a pair of heads. The data is
written with a thin film inductive head and read
with a Giant-Magnetoresistive (GMR) head, a
descendant of the Magnetoresistive (MR) Head.
3. ACTUATORS
(b)
(a)
(c)
as to most closely mimic the motion of the linear
actuator. As drives shrank once more, the actuator pivot was put in one corner of the enclosure.
The sideways turn in the suspension was removed
so that the actuator and suspension were in a
single line being dragged over the disk. As actuator sizes have shrunk, the structural resonances
have moved to higher frequencies. In contrast, the
effects of friction in the pivot bearing have become
more noticeable, as discussed in Section 5.6.
(d)
4. SERVO SIGNALS
50 Disks
Fig. 4. Generalized view of the evolution of actuators. Note the transition from the RAMAC
(a) to the comb structure which had one head
per surface (b). The next transition is from
linear to rotary (c), then from larger rotary
to smaller stiffer rotary actuators (d).
The original RAMAC actuator used aircraft cable
and pulleys, as did the system that Hagopian
describes in (Hagopian, 1961), but the next products and on through the IBM 2314 – the main
product until the IBM 3330 Merlin drive was
introduced in 1971 – all used hydraulic actuators.
Hydraulic actuators offered far better seek performance than magnetics, but could not provide a
track following servo and also had a problem with
contamination due to oil leaks (Oswald, 2001).
The original RAMAC actuator had a single pair
of heads that moved both vertically and radially
to access data (Figure 4a). The heads floated on
a static air bearing which required a separate
pumping mechanism. The invention of the self
actuating air bearing allowed there to be one slider
per surface and led to the comb actuator. By 1965,
these were driven by a linear voice coil motor (Figure 4b). The next major step was a move to rotary actuators, first designed at IBM’s Winchester
Labs (in Winchester, England) (Porter, 2000).
The IBMers in England were associated with
IBM’s Rochester disk drive group – working on
drives for minicomputers and smaller systems, so
they had a different approach than the folks in
San Jose. The early Rochester drives (Spartan
and the IBM 9332) had linear actuators. However,
people at IBM’s Fujisawa operation were looking
at rotary actuators. When they put the rotary
actuator drives on shaker tables, they found that
those drives had much better immunity to linear
shock and vibration.
The rotary actuators for 8” drives were relatively
large and complicated mechanical truss structures. They were large due to the size of the disk,
but the trusses minimized the inertia of the actuator. The suspension was turned sideways on these
The RAMAC had a servo system to move the
actuator vertically and radially. The positions
were “detected” by detent marks on the actuators.
The early comb actuators were hydraulic until
IBM introduced the first voice coil motor in a
drive in 1965 (Stevens, 1997). These were open
loop until 1971 when IBM introduced the first
disk drive that closed the loop with position
information read from the loop (Oswald, 1971;
Oswald, 1974), although the idea was studied
much earlier (Hoagland, 1961).
A variant of the off-disk position information was
found in early drives made by Quantum which
used an optical encoder (stuck to the side of
the VCM) as a servo mechanism (up to trackdensities of around 2100 TPI). They worked well,
and did not require a servowriter. They of course
suffered all of the disadvantages of dedicated
servo systems, plus the disadvantage of having
a position reference that was completely off the
disk (Ehrlich, 2001).
Modern disk drives read the relative position of
the head to the track directly from the disk media.
Over the history of closed loop control of disk
drives there have been two essential choices for
encoding this position information: dedicated and
embedded (or sectored). Dedicated servo involves
reserving an entire disk surface for position information, leaving the other surfaces free to contain
only user data, as shown in the left side of Figure 2. Embedded servo time multiplexes the servo
information with the user data on each surface as
shown in the center diagram of Figure 2. Dedicated servos have the advantage of higher sample
rates and a possible savings in surface area when
the total number of disk surfaces is relatively high.
On the other hand, they are inappropriate for
single surface systems, poor choices for single disk
systems and typically have more susceptibility to
thermal offsets than embedded servos. In order to
minimize the effects of thermal offsets, the servo
information on a dedicated servo system is usually
encoded on one of the center surfaces as shown in
Figure 2. Embedded or sectored servos, as shown
in the center diagram of Figure 2, co-locate the
position sensing with the control, but force the
servo designers to choose between higher sample rates (desirable) and lower user data density
(undesirable). However, as track densities have
increased, the thermal offsets in the head stack
assembly have become too large a percentage of
the track to do anything other than embedded
servos (co-located control).
A variety of position encoding methods have been
used to encode the servo position including amplitude, phase (Boutaghou et al., 1994) , frequency,
and null (Wilson, 1994; Sacks, 1995; Sacks et
al., 1996) encoding. Figure 3 shows a diagram
of amplitude and null encoded servo patterns.
The servo signals must be demodulated into the
baseband to be useful. Over the course of disk
drives’ history, the methods have trailed those of
the data channels. Peak detection channels have
been used to detect position information up until
very recently (Silicon Systems Staff, 1994).
5. THE CONTROL PROBLEM
With all the above background plus that in
(Abramovitch and Franklin, 2002), we can now
delve into the history of the disk drive feedback
loops themselves. Far from being an isolated problem, the control loops were tightly coupled to
all the component technologies that were being
developed. Limits on the ability to improve the
drive’s track density would prompt changes in the
magnetics or mechanics of the drive. Through all
of this, the bit aspect ratio, that is the track
width of a bit compared to the down the track
length of a bit, has stayed centered around 20 to
1 (Abramovitch and Franklin, 2002).
The first commercial hard disk to read the relative
position of the read/write head from the disk
itself and close a feedback loop around this was
IBM’s 3330 Merlin drive, which came out in 1971.
However, it was IBM’s 3340 Winchester drive
which shipped in 1973 that set the architecture for
future disk drive control loops. The Winchester
drive is mechanically significant for its use of
lightly loaded, lubricated, low-mass sliders, but its
control system is most significant in that it is the
first drive in which all the pieces of a disk drive
control loop were in place. The paper written by
Dick Oswald, a lead servo engineer on the project,
has been the classic starting point for disk servo
engineers (Oswald, 1974). It was common practice
at some disk drive companies for an engineer to
go to their first day at work only to find a copy of
Oswald’s paper on their desk (Hurst, 2001).
The 3340 Winchester drive put together many
ideas that had been floating around for a while.
The idea of reading a signal from the magnetic
medium and using this for fine positioning appears
to be discussed in a paper by R. Tickell at the
1966 Joint American Conference (Tickell, 1966)
and in an earlier paper by Al Hoagland (now
at Santa Clara University) in the IBM Journal (Hoagland, 1961). Tickell worked at Sperry
Rand in the Univac Division which had put off
coming out with a disk drive product in 1956
in favor of magnetic drum storage (Abramovitch
and Franklin, 2002). Thus it was that Tickell’s
paper closes the loop using signals read back from
a magnetic drum – not a disk. Still, the paper
introduces some concepts that would be used by
Merlin and future disk drives. Rather than use
each of the data heads to read the track position,
this system uses a single pair of heads to read a
dedicated position track – the first dedicated servo
head. A pair of heads is used to get common mode
rejection of media variations. Furthermore, the
positioning is segmented between track numbers
(detected digitally) and the sub-track position
(detected with the servo head).
Hoagland’s paper (Hoagland, 1961) discusses a
team project to build a laboratory setup that
did fine positioning of a hydraulic actuator on a
magnetic disk by reading magnetically encoded
position fields off the interior area of the disk.
It also introduces a set of concepts that would
be used in the IBM 3340. A set of data heads is
ganged together with a servo head and a sector
head to determine the fine position. Multiple
position pulses are averaged together to provide
some immunity from noisy or bad pulses. The
concept of using a dual element head, one to read
data and one to write data shows up here as well,
allowing the write wide, read narrow procedure
that is fairly common in today’s drives that use
MR heads.
The actual position encoding method in the
3340 was a tri-bit encoding method proposed
by Mueller (Mueller, 1972). It won out over the
method used in the IBM 3330 Merlin, in which alternating position information tracks had magnetization patterns with opposite polarity (Santana,
1970). IBM had also been working on average
seek time studies (Hertrich, 1965) and on the
use of bang-bang control for seeks (Brown and
Ma, 1968). However, in Oswald’s paper a modification of the bang-bang scheme, which would later
be codified as a Proximate Time Optimal Servo
(PTOS) (Workman, 1987b) was first discussed.
With all of these coming together, one can see the
architecture of future disk drive control systems
in Oswald’s paper.
There is an old saying in the computer industry
which dates back to the days when IBM had a virtual monopoly on computing. In those days it was
said that if IBM published something, it meant
they had decided not to use it 1 . Information
available for this section often has a similar flavor.
Most disk drive companies are fairly tight lipped
about their servo work and thus much of the history that follows is from published work, patented
inventions, consortium meetings, the work of companies no longer in the industry, work with which
the authors have direct knowledge, and/or industry rumors. Although this provides a rich tapestry
of work to draw from, the reader is cautioned
against thinking that this is complete. There is
a lot that the drive companies are just not telling
us. As for the publications, they tend to be either
the result of industry-academic collaboration or
industry patented work.
Another phenomenon that keeps the servo work
secret is that by and large disk drive companies
have cross license agreements on all their patents.
As their competitors have ready access to their
patents, many disk drive companies shy away from
patents in favor of keeping information as a trade
secret. Trade secrets don’t get published while
patents are public information.
Finally, disk drive companies and servo engineers
are usually risk averse. This tends to make them
extremely slow to pick up new algorithms, much
to the chagrin of their academic collaborators.
The general feeling is that a new piece of technology gets added slowly as the risk of that technology goes down and when some change in a
component around the loop changes the balance
of the PES composition.
With this caveat in mind, the rest of this section
will delve into specific concentrations of work on
hard disk servos.
5.1 Analog, Sampled Data and Digital Control
While the RAMAC used tube electronics (Stevens,
1997), drives progressed rather quickly to transistors and integrated circuits. From the beginning,
the drives had digital circuitry to relay the data to
and from the computer. Early digital controllers
on drives emerged for reasons of either economy
or performance.
An example of using digital control for economic
purposes came from Quantum, which had some
low end drives with computer interface microprocessor. By using this processor to do servo control,
they were able to save on the cost of analog electronics. IBM on the other hand, was working to
implement advanced algorithms on their drives for
minicomputers (Ottesen, 1988; Stich, 1987) and
mainframes (Franklin et al., 1990).
1 Thanks go to Dennis Bernstein for triggering this memory.
It is worth understanding that the very nature
of encoding position on a disk drive implies a
sampling process. Thus, even when the control
laws were implemented using analog electronics,
the system was a sampled data system. For dedicated servo drives, this sample rate was often
high enough to masquerade as continuous position
information passed through a low pass filter. However, sectored servo pushes sample rates lower,
forcing the control system designer to deal with
the sampling of their system. HP had drives of
this nature through the mid 1980s. Some of the
quirky aspects of this type of a system are touched
on in a paper on frequency response function measurements (Ehrlich et al., 1989). It was not until
the late 1980s that HP moved to fully digitally
controlled drives, in large part due to the interaction between Rick Ehrlich (then at HP Labs) and
Vernon Knowles at HP’s Disk Memory Division.
It is worth noting that IBM’s disk drive operations
were originally tied to their various computer operations and thus tended to take on the culture of
the computer operations rather than being monolithic within the drive operation itself. This was
particularly evident in different levels of openness
with the rest of the industry between San Jose and
Rochester. Rochester, Minnesota is most famous
for the Mayo Clinic, but it was also the site of
a major IBM hard disk facility. In its heyday,
this site made drives for the minicomputer market. Rochester, being tied to the minicomputer
business which competed with DEC, had a relatively open attitude. San Jose, involved with the
mainframe industry in which IBM had a virtual
monopoly, tended to be more closed.
Rochester was competing in the small form factor
market, so they were more outward looking and
open than San Jose. They would buy cameras
and consumer electronics and take them apart to
see how those products did packaging. They were
always looking for ways to get things into smaller
form factors. It’s likely that if the Rochester folks
had been in San Jose, they wouldn’t have been
able to do what they were doing. Their distance
from San Jose allowed them to be the “rebels in a
cornfield” (Ottesen, 2001). This is an interesting
irony, in that it was San Jose’s distance from
IBM headquarters in Armonk that had given them
the freedom to work on the RAMAC almost 3
decades earlier. By the mid-1980s San Jose was
the entrenched big brother; Rochester the strident
kid brother. Thus, Rochester management was
somewhat antagonistic to San Jose management,
whereas the latter tended to ignore the upstarts.
Thus, IBM San Jose was producing 14” disk drives
late into the 1980s, long after the rest of the
industry had moved to smaller form factors.
When Rochester folks proposed digital control,
they got laughed out of the room by San Jose
folks. Thus, it was the gang at Rochester that
came up with IBM’s first digital control drive,
code named Spartan. This was an 8” drive intended for mini-computers. They had an advanced
development lab in Hursley, England and a development lab next door in Winchester, England.
It was the English team that came up with the
sectored servo approach that is currently used in
virtually all disk drives.
Another interesting start for digital control was
relayed by Fred Kurzweil. Fred, who has the
distinction of being Gene Franklin’s first graduate
student at Stanford, graduated in 1959 and went
to work for IBM Research in San Jose. After 23
years of doing mostly theoretical work on disk
drive control, he took early retirement and went
to Maxtor which had just opened its doors in June
of 1982. Upon arriving at Maxtor, he was thrown
into the fire of having to make the first in-thehub spindle controller work. Upon hearing stories
circulating about this project, industry pioneer Al
Shugart said it would never work. Folks around
the industry listened to Shugart eventually giving
Maxtor a 2 year head start in this technology.
Arriving at Maxtor in October of 1982, Fred
was one of the few people there who knew all
about electric motors. Putting the motor in the
spindle hub raised some serious issues. The motor
consumed 10 watts of power, so being inside
the hub, it tended to heat up. This meant that
conditions were different at a cold start when
the grease around the spindle ball bearings was
cold than once the hub heated up and the grease
flowed more freely. Fred solved this problem with
a 10 cent microprocessor and some simple digital
control. In order to maintain the spindle speed to
1 part in a million, he used simple adaptation: he
would measure the speed over a revolution and
perform a very low order correction so that as
temperature affected the grease in the bearing (it
started running faster as things got warmer) the
speed stayed constant. Another issue was that the
motor was a 3 phase induction motor. They had to
get the motor up to speed before they could get a
feedback signal from it to close the loop. They ran
into stiction problems at start up. Often the motor
didn’t start and they had to dither the input to
break it free (Kurzweil, 2001).
All these neat control ideas were tried on the
spindle controller. However, this microprocessor
was not suitable for high speed digital control
and thus the rest of the drive used analog control
loops.
5.2 State Space
With the emergence of DSPs and digital control
on hard disks, the possibility of doing state space
control became more real. Early disk drive digital
control systems used classical design methods.
The first use of state space control seems to
have been during seek mode. There are a couple
of reasons for this. First of all, the number of
operations for a state space controller are typically
larger than those for a classical controller on
the same order problem. Early DSPs were hard
pressed to do all the extra operations in a single
sample interval while track following. The second
reason is that a measurement of the back EMF
from the voice coil motor – which could be used
to estimate velocity – was only useful in seek mode
when the signal was large, i.e. during seek.
Classical design techniques lend themselves to
being able to directly use the measured frequency
response functions, without having to compute a
parametric model. Parametric models that closely
match these frequency response functions often
have on the order of 30 modes (Hanselmann and
Engelke, 1988). Thus, if a designer wants to use
state space techniques, they must either design
with a high order model and controller, or go
through an often lengthy model reduction step to
capture the essence of the dynamics.
The use of state space control to make use of
the back EMF sensor during seek while using a
classical design in track following was used by
Quantum into the late 1980s and at HP until they
got out of the drive business in 1996 (Knowles,
1991).
On the other hand, IBM Rochester got into the
state space paradigm early on. Rochester started
on digital control in 1980. They had gotten a
microprocessor from Intel that was just released.
A military processor that had DSP capability
(the Intel 8196). Mike Stich, a servo engineer
out of Rochester, took the digital controls class
at Stanford from Gene Franklin through fellow
professor Marty Hellman’s company. Franklin and
Powell’s first digital control book, the “little blue
book” (Franklin and Powell, 1980), had just come
out. After Stich went back to Rochester, he contacted Franklin to see if he was interested in
teaching at Rochester. Ottesen had a copy of
the little blue book and handled the logistics of
having Franklin come to IBM Rochester to do
some consulting and teach some digital controls
classes.
The IBM 9332 out of IBM’s Rochester operation
was not only the drive with the first 1-7 RLL
code (IBM Storage Technology Division, 2001),
but also made use of a digital state space controller that even did on-line parameter adapta-
tion (Ottesen, 1988; Stich, 1987; Stich et al.,
1987). IBM Rochester, had relatively little pain
going into state space. There was nobody around
that had any experience with digital control so
they were on their own to experiment as they
thought best. Back then they used APL, a cryptic
computer language at best, to model their systems (Ottesen, 2001).
The case history on disk drive control written
by Mike Workman (of IBM San Jose) for the
digital control book by Franklin, Powell, and
Workman (Franklin et al., 1990) makes use of
state space control as well, indicating that IBM
San Jose had fully embraced the state space
approach as well, but after Rochester did. In
fact, Hal Ottesen, a long time servo engineer at
IBM Rochester recalls that when the folks from
Rochester first proposed digital control to the
folks at San Jose, they were laughed out of the
room (Ottesen, 2001). IBM San Jose’s move to
digital and state space control was largely led by
Mike Workman, who had been taking classes at
Stanford and gotten “the religion”. It was largely
through the influence of Mike Workman that IBM
San Jose moved into the digital control and state
space world. Their first digital control drive was
the IBM 3380K.
Hal Ottesen would travel around IBM sites teaching internal classes on digital and state space control of disk drives. His hand written notes evolved
over time, but have never been published.
According to Rick Ehrlich, Quantum’s first drive
to use state-space for seeking and tracking, and
with embedded servo was the high-end Enterprise
drive which went into mass production in early
1992. The desktop drives went to single-loop statespace control a couple of years later (when the
microprocessor was fast enough that they didn’t
have to drop to a simple servo in ontrack mode to
make time for the I/O firmware) (Ehrlich, 2001).
Currently, it is believed that most if not all
disk drive controllers these days are state space
controllers.
5.3 Sample Rates
The multiplexing of position information with
user data on hard disks creates a set of competing
objectives. On one side is the desire for maximum
data storage which would push to minimize the
number of servo fields within a track. On the
other side is the desire for improved performance
in the control system which often requires a higher
sample rate. These tradeoffs have limited the
achievable sample rates to the range of 6-14 kHz.
This in turn has limited the achievable tracking
closed loop bandwidth to the range of 500-1000
Hz.
This has led to a fair amount of work in multirate servos where the control output is changed at
a significantly higher frequency than the sample
rate of the PES. Rick Ehrlich had been pursuing the idea of multi-rate control ever since he
was at Hewlett-Packard (Ehrlich et al., 1989).
Carl Taussig had continued this work, coming
up with unpublished results that were quite similar to those reported by W. W. Chiang (out
of IBM Almaden Research Center in San Jose)
in 1990 (Chiang, 1990). It turns out that Rick
Ehrlich had continued his multi-rate work at
Quantum and was getting similar results. Basically, if the Position Error Signal (PES) sample
rate was relatively low – say under 8 times the
open loop crossover – then keeping everything else
constant and raising the output sample rate by a
factor of 3 or 4 over the input sample rate could
result in a closed-loop bandwidth improvement
of roughly 20%. If, on the other hand, the PES
sample rate was already at roughly 20 times the
open loop crossover, then the improvement was
far smaller.
5.4 Repetitive and Spectral Disturbances
One of the concepts to hit disk drive control systems at the end of the 1980s was that of repetitive control to cancel the effects of the spindle
eccentricity. This started initially in Masayoshi
Tomizuka’s group in the Mechanical Engineering
Department at UC Berkeley. Tomizuka’s group
was working on practical applications of repetitive
control as a solution to repetitive disturbances in
rotating machinery. Obtaining some disk drives
from IBM, but unable to access the drive DSPs to
change the code, they used the notion of an add-on
controller that would augment the nominal loop
to remove the harmonic disturbances (Tomizuka
et al., 1988; Chew and Tomizuka, 1989; Kempf et
al., 1993; Chew and Tomizuka, 1990). As graduate
students branched out from Berkeley, especially it
seems to professorships at CMU (e.g. Marc Bodson who later moved to Utah and Bill Messner),
these studies also included adaptive feedforward
harmonic cancellers, which were shown to have
some equivalence with repetitive controllers. As
the 1990s would progress, the use of harmonic
correctors would become standard throughout the
industry.
5.5 External Shock and Vibration
As drives became smaller and moved to more
mobile applications in the early 1990s, the issue of
rejecting external disturbances, namely shock and
vibration, became more prominent. An enabling
factor in this was the continuing drop in the cost
of accelerometers to the point at which one could
reasonably consider them as an option for disk
drives.
However, the earliest examples of accelerometer
control in a disk drive go all the way back to
the 1970s. White used an accelerometer to sense
shock and then minimize the probability of the
heads slapping against the magnetic media by
1) increasing the air pressure within the drive
to stiffen the air bearing and 2) simply unload
the heads from the disk surface (White, 1977).
There was also some work by Robert Smith at
Seagate (Smith, 1993).
Typical use of accelerometers involves sensing the
disturbance and moving the actuator before the
error ever shows up in the position error signal.
As the accelerometer and drive characteristics are
subject to change, adaptive methods are often
used. Generally speaking, the use of accelerometer feedforward dramatically improves the disturbance rejection capabilities of hard disks. Cost
and reliability issues for the accelerometers themselves limited this practice to the lab through the
early 1990s.
An exception to this occurred in the late 1980s.
Hewlett-Packard was making use of a center of
percussion rotary actuator as opposed to the more
common balanced actuator. This left the drive
susceptible to translational shocks and so two
of the servo engineers, Vern Knowles and Mitch
Hanks made use of a linear accelerometer to generate a feedforward compensation signal (Knowles
and Hanks, 1987). However, each of these had
to be calibrated to the disk in the factory, thus
raising the manufacturing cost.
Davies, an MIT graduate student, and Mike Sidman, an engineer at Digital Equipment Corporation’s Advanced Storage Lab in Colorado Springs,
formulated conditions by which an accelerometer
could perfectly cancel external and internal disturbances (Davies and Sidman, 1991; Davies and
Sidman, 1993). Not long after this, Sidman left
Digital and the work was not pursued.
running a swept sine measurement. Apparently,
the technician accidentally set the lower sweep
frequency to 10 Hz instead of 100 Hz. The resulting frequency response function did not look like
anything described by their existing models and
the study of disk drive friction was on.
Starting in the early 1990s, there were several
publicized efforts to analyze and mitigate friction.
Work was being done at Quantum in 1991 to
characterize the nature of the ball bearing actuator pivots of their 3.5” drives. Mike Hatch
and Bill Moon used time domain measurements
to establish a hysteretic relationship between position and velocity of the actuator which they
reported at an early National Storage Industry Consortium (NSIC) meeting in La Jolla in
February of 1992. Hewlett-Packard would struggle
with friction issues on the KittyHawk project, a
1.3” drive (Abramovitch et al., 1994; Wang et
al., 1994). Friction mitigation work would continue sporadically around the industry through
the 1990s.
5.7 Dual Stage Actuators
The success of dual stage actuators in the tracking
loops of optical disks has led to many proposals
to add these to hard disks. The reasons for doing
this are:
• Lowered inertia for higher bandwidth actuation.
• Lowered energy power requirements for high
bandwidth.
• Mitigation of pivot friction effects.
• Putting high bandwidth actuation at the
end of the actuator beyond the effect of the
suspension resonances.
However, most of these projects would not get
going until the middle of the 1990s. By the year
2000, there would be predictions of universal acceptance of dual stage actuators even though none
had shown up in any products (Stevens and DeLillis, 2000). The burgeoning field of micromachining
would be the chief technology for most of these
designs.
5.6 Friction
Friction is an issue whenever two surfaces in
contact move relative to each other. In hard disks,
friction in the rotary actuator pivot is an ongoing
issue studied by both the mechanical and servo
portions of a disk drive team. The issue becomes
more noticeable as the actuator inertia drops and
thus is more of an issue for small disk drives.
The story goes that friction in disk drives was
discovered accidentally when a lab technician was
5.8 Seeks
The first drive that closed the loop based on
position information read from the disk was IBM’s
3330 Merlin drive. The Merlin drive attempted use
of a bang-bang controller. However, the control
system was superseded by that of the IBM 3340
Winchester, described in the seminal (but short
paper) by Dick Oswald (Oswald, 1974). In this,
he describes not only the track following operation, but also a near time-optimal seek mechanism. It turns out that IBM had been working on this for a while (Brown and Ma, 1968),
and this technique would become standard practice in the industry. However, it was not until
Mike Workman (an IBM engineer turned manager) wrote his doctoral thesis at Stanford while
in Gene Franklin’s group that this method would
be codified as Proximate Time Optimal Servomechanism (PTOS) (Workman, 1987b; Workman, 1987a; Workman et al., 1987a; Workman et
al., 1987b). The adaptive version was named APTOS, in part because it reminded his wife, Patti,
of a town in the San Francisco Bay Area. Further
work to extend PTOS beyond the saturating double integrator and into flexible structures was pursued by Lucy Pao, another student of Franklin’s
at Stanford (Pao and Franklin, 1990; Pao and
Franklin, 1992; Pao and Franklin, 1993; Pao, 1994;
Pao and Franklin, 1994).
One of the issues with seeks is that the reference
positions given to the control system often excite
the flexible modes of the actuator, even in closedloop. This residual vibration takes a long time to
damp out and effectively lengthens the seek time,
since data cannot be read or written until the head
is settled to within some small fraction of a track
(typically under 10%). Neil Singer, a graduate student at MIT under Warren Seering, developed a
method called command input shaping, for use in
flexible robotics (Singer and Seering, 1988; Singer
and Seering, 1989). The idea is to prefilter the
reference command in such a way as to remove
spectral components that will stimulate the residual vibration modes of the actuator. This results
in a response with slightly slower rise times, but
much faster settling times and was applied to
NASAs space shuttle manipulator arm. In any
event, it turned out that Neil Singer was a friend
of Carl Taussig, who was at that time working on
disk drive research at HP Labs. A collaborative
effort ensued to apply this technique to disk drive
manipulators.
5.9 Mode Switching Control
One of the aspects common to virtually all disk
drive control systems is the notion of mode switching control, where the control system switches
from one mode of operation to another. As a drive
initiates a seek, it switches in the seek control algorithm. Typically these mimic bang-bang control
at least during the acceleration phase. During deceleration, the drive follows a velocity profile into
the target position, then switches to settle mode,
and finally into track following mode. Quite often,
the latter involves switching an extra integrator
into the control loop.
During the early days of disk drives, this mode
switching was accomplished by switching in different electronics for each region. As drives went
to digital control, they did not switch electronics,
but the control law was switched in the microprocessor. A salient feature of then DEC disk drive
servo engineer Mike Sidman’s doctoral thesis was
a smooth transfer between seek, settle, and track
following mode (Sidman, 1986).
While the seek algorithms were codified by Mike
Workman in his original work on PTOS (Workman,
1987a), it seems that even today, most disk drive
servo systems still go through at least 3 stages of
control. HP’s disk drives of the early 1990s had
5 modes during seek, then a flare, gross settle,
and settle mode, before moving to track following
mode (Knowles, 1991).
The early IBM digital control drives mimicked
the mode switching of analog controllers. The
seek control followed a velocity profile. Settle
mode used fairly stable poles. Once track following
mode was switched in, an extra integrator was
added to the control law computation. Furthermore, computational delay from position measurement to control output was modeled. Finally,
they waited longer in settle mode if the drive was
about to write data than if it was going to read
data (Ottesen, 2001).
6. THE 1990S AND BEYOND
The 1990s would see several major trends. The use
of MR heads would increase through the decade
to become standard. The heads themselves would
go through several generations to the present day
Giant Magneto-Resistive Heads (GMR).
Drives would get smaller as areal densities increased at 60% a year resulting in small drives
with massive capacities. Unfortunately for the
industry, hard disks would become commodity
items, limiting profits of the companies.
DSPs would become standard in hard disk servo
systems. Servo system would move to state
space. Harmonic correctors would become standard practice. Spindle speeds would rise, pushing bandwidths and mechanics. This in turn
would put pressure on improving the servo signals (Abramovitch and Franklin, 2002).
In the 1990s, most drive companies would move
their production offshore. Most of these chased
cheap labor in Southeast Asia. These moves did
not hold off the consolidation in the industry.
DEC would sell off what was left of their disk
drive business to Quantum, who would sell off
their disk business to Maxtor. Conner Peripherals
and Imprimis would be absorbed into Seagate.
IBM would shut down its Rochester operations.
HP would exit the business completely.
ACKNOWLEDGEMENTS
It is impossible to write such a paper as this without soliciting and receiving a considerable amount
of assistance. P.D. Mathur of Seagate, Dick Curran and Fred Hansen of Quantum/Maxtor, and
Ed Grochowski of IBM for very useful historical
data on their companies drives. Dick Henze and
Chuck Morehouse (HP) as well as Bob Evans
(Hutchinson Technology) provided useful technical data at key points in the writing. Rick Ehrlich
(Quantum/Maxtor), Terril Hurst (HP), Lucy Pao
(U. of Colorado), Ho Seong Lee (Maxtor), and
Masayoshi Tomizuka (UC Berkeley) provided not
only lots of useful information, but also many
helpful suggestions on improving the paper. Dick
Oswald, Fred Kurzweil, Hal Ottesen, and Jim
Porter all provided fascinating stories on the early
days of disk drive control. Finally, the gathering
of old and sometimes obscure references for this
work would not have been possible without the
tireless assistance of the Agilent Labs Library
research staff, particularly Ron Rodrigues and
Sandy Madison. To all of these people, we owe
a debt of gratitude.
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